The head-up display (HUD) performs one of the most critical and complex functions of any avionics system in modern combat aircraft, as well as in other state-of-the-art aircraft. Display of visual data on the HUD allows the pilot to control the aircraft, navigate, target and perform other key tasks during an engagement without removing or refocussing his eyes from the frontal view of the aircraft. In a HUD, pilot information is projected from a cathode ray tube (CRT) through relay optics onto a transparent reflector, normally referred to as a combiner glass, positioned between the pilot and the windscreen of the aircraft.
Originally, combiners were broadband partially mirrored glass, because such broadband reflectors are easily implemented reflectors of CRT light. Unfortunately, when they are sufficiently mirrored to reflect a sufficient amount of CRT produced light for high ambient light conditions, they also attenuate light from the outside world that should be transmitted therethrough to an unacceptable degree.
The current performance requirements of HUD combiners for combat aircraft include: high photopic transparency and high reflectance efficiency at the wavelengths of the phosphors of the CRT. Heretofore, such requirements have dictated the use of dichromated gelatin holographic thin films as the active combiner material.
Dichromated gelatin holographic thin films are difficult to fabricate, requiring large capital investments and long learning times for manufacturing personnel as the process is as much art as science. Therefore, there are only a few suppliers of such dichromated gelatin combiners in the entire world, and those that can produce them, do so at a very high cost. Such hologram fabrication is inherently a sequential "one step at a time" process. The resultant gelatin thin films are very fragile and highly susceptible to mechanical abrasion and moisture damage so dichromated gelatin combiners must be protected by heavy, cemented cover glass on the opposite sides thereof and a circumferential seal. These difficulties make holographic combiners extremely costly, and subject to long lead times and occasional unavailability. In addition, optical noise recorded during the holographic recording process produces flare or rainbow patterns around point sources viewed through the combiner, reducing visual acuity.
Because of the inefficiencies of available combiners, CRTs must be run at extremely high intensity levels in locations where little space is available for cooling. The result is the CRTs must be replaced after as little as 100 hours of daylight operation. The CRT usually must be mounted forward of the instrument panel in a crowded nose area under the windscreen. Therefore, CRT replacement is very time consuming, resulting in unavailability the aircraft for a substantial length of time while its HUD CRT being is replaced and contributing in a major way to the number of maintenance hours required for every hour flown.
Holographic combiners have high visibility through the elements but are inefficient CRT phosphor reflectors due to their limited angular bandwidth. Optical power can be added to increase angular bandwidth, but only at higher and higher optical noise levels. Superposition of two or more holograms can be used to roughly approximate phosphor spectral output, but such approaches also have limited flexibility, much higher cost, and increased combiner noise.
Therefore, there has been a need to improve optical performance of aircraft head-up display combiners while simultaneously reducing cost and weight.